1. Field of the Invention
The present invention relates to an epitaxial wafer used in a semiconductor light emitting device such as a light emitting diode or semiconductor laser as well as to a semiconductor light emitting device fabricated by using the epitaxial wafer.
2. Description of Related Art
The metal organic vapor phase epitaxy (MOVPE) method has been mainly used as a growth method of a crystal used in a semiconductor light emitting device. When a semiconductor crystal including compounds in the III and V groups is grown in this method, a substrate disposed in a vapor phase epitaxy apparatus is heated, after which a III-group metal organic raw gas, V-group raw gas, carrier gas, and dopant raw gas, which are used as raw materials of epitaxial layers, are supplied into an epitaxy furnace. Then, the resulting mixed gas is thermally decomposed in the furnace so that crystalline films are grown and stacked.
The epitaxial wafer for a semiconductor light emitting device obtained by the above MOVPE method is used in various types of semiconductor devices. Examples of these semiconductor devices include semiconductor laser diodes (LDs) used as light sources in read and write operations in optical disk systems, and light emitting diodes (LEDs) used in many applications including displays, remote controllers, sensors, and lamps in cars.
When an AlGaInP (aluminum-gallium-indium-phosphorus) crystal for a semiconductor light emitting device is grown in the MOVPE method, Zn (zinc) and Mg (magnesium) are generally used as p-type dopants. In particular, the Mg is often used in the p-type AlGaInP cladding layer in a compound semiconductor crystal for a high-output LD. This is because the Mg has a smaller diffusion coefficient than the Zn and is thereby hard to diffuse into the active layer, so the Mg can be doped at a relatively high concentration.
In the p-type contact layer, electrode contact resistance needs to be lowered as much as possible, so the carrier density in that layer needs to be higher than in the cladding layer by at least one order of magnitude. Accordingly, the p-type contact layer is generally formed with gallium arsenide (GaAs), and the Zn is used as the dopant. The Zn can be doped into GaAs at a high concentration of about 1×1019 cm−3, which is higher than the Mg can be doped.
On the other hand, light sources used for read and write operations in high-density optical disk drives are required to achieve a stable high-output operation even at high temperatures. To meet this requirement, the carrier density of the p-type cladding layer in the epitaxial wafer for a semiconductor light emitting device must be increased to about 1×1018 cm−3. In this case, the Mg doping is advantageous in less diffusion at a concentration order of 1018 cm−3 because of its small diffusion coefficient, as mentioned before.
When the p-type cladding layer to which the Mg is doped is used, however, mutual diffusion of the Zn and Mg is prone to occur significantly; the Zn in the p-type contact layer diffuses up to the p-type cladding layer and active layer during the p-type contact layer is growing. As a result, the full width of half maximum of the photoluminescence spectrum in the active layer becomes large, causing critical problems such as deterioration in light emitting characteristics in the active layer and reduction in lifetime.
In order to solve these problems, various methods of preventing Zn from diffusing have been proposed. To prevent the Zn from diffusing from the substrate into the active layer, a method is disclosed in JP-A-2002-111052, in which a Zn diffusion preventing layer into which both the Zn and Si (silicon) are doped is provided between the substrate and the p-type cladding layer. In another method of preventing the Zn from diffusing from a p-type cap layer (p-type contact layer), which is disclosed in JP-A-2006-19695, the p-type cap layer is formed as at least two layers, which are an Mg-doped layer (which functions as the Zn diffusion preventing layer) and a Zn-doped layer stacked thereon when viewed from the substrate. In still another method, which is disclosed in JP-A-2007-96267, a p-type AlGaAs layer (which functions as the Zn diffusion preventing layer) into which C (carbon) is doped is inserted between the p-type cladding layer and the p-type cap layer.
According to the methods in JP-A-2006-19695 and JP-A-2007-96267, Zn diffusion from the Zn-doped p-type contact layer into the Mg-doped p-type cladding layer and undoped active layer is extremely efficiently suppressed to an extent at which there is almost no Zn diffusion.
If, however, the Zn diffusion from the p-type contact layer into the p-type cladding layer and the like is almost suppressed by the above-mentioned conventional methods, when a conventional device fabrication process has been tuned on the assumption that Zn diffusion occurs, the process may need to be significantly modified. In some cases, appropriate Zn diffusion may be effective to stabilize semiconductor light emitting device characteristics. Accordingly, if Zn diffusion from the p-type contact layer into the p-type cladding layer and the like is almost suppressed by the above conventional methods, a problem that semiconductor light emitting device characteristics become unstable arises.